Research in solid-state lighting and in particular on new phosphor materials for white-light emitting diodes (LEDs) can provide substantial energy savings and reduce our global environmental impact. Phosphors are materials capable of emitting photoluminescence when excited by radiation. They are used in LEDs, cathode ray tubes and sensors. LEDs are extensively used for applications in industrial and domestic lighting, indicator lights, traffic signs, automotive dashboards, and portable flash lights. Phosphors for lighting applications can be made in two ways: doping with activator ions or self-activation. Activator doped phosphors are prepared by doping activator ions in small concentrations into a host material. By direct excitation with light or by subsequent energy transfer, electrons populate the excited energy levels of the activator ions, and when they revert to the ground state, light emission referred to as photoluminescence takes place. The emitted radiation can have higher (“up-conversion”) or lower (“down-conversion”) energy than the energy of the excitation light. In another type of phosphor termed self-activating, the photoluminescence, is based on electron activation within subunits of the structure. Self-activating CaWO4 has a blue emission band centered at about 420 nm which is caused by charge transfer within WO42− complexes, where the oxygen 2p electrons partially occupy the empty 5d orbitals of tungsten. Rare earth activated phosphors show line emission when excited by UV radiation, which causes the electrons to absorb photons and excite them to higher energy levels. Subsequently they relax back to the lowest vibrational energy level of the first excited state before finally returning to the ground state by emitting energy in the form of light. A broad band emission occurs when there is a shift in the equilibrium position upon excitation by photon absorption.
The most widely used phosphors for LEDs currently are oxides (e.g., Ce3+-doped Y3Ga5O12, Lu3Al5O12, Gd3Sc2Al3O12 and Ca3Sc2Si3O12), nitrides (e.g., Sr2Si5N8, SiN4), or sulfides (e.g., SrS:Eu2+, SrGaS4:Eu2+. Y3Al5O12:Ce3+ (YAG:Ce3+)) that are used as a phosphor for blue-emitting InxGa1−xN based LEDs as it efficiently absorbs part of the blue light and then emits bright yellow light to create cold white light.
Disadvantages associated with existing phosphors such as sulfides, nitrides and fluorides triggered the search for alternative materials with photoluminescent properties among them oxyfluorides. Many phosphors are not very stable chemically when exposed to air and humidity. This can lead to the emission of corrosive gases such as H2S and/or HF. Most phosphors undergo significant thermal quenching, e.g. Sr2SiS4:Eu2+ has a T1/2 value of 380 K, which is the temperature where the emission intensity becomes half of its value at room temperature. Thermal quenching processes occur when electrons in a material are excited and relax back to the ground state by nonradiative transitions since the excited state and ground state energy parabolas cross energies thermally accessible from the excited state. The excited electron reaches the ground state without emitting light and the photoluminescence intensity decreases as a consequence. Although nitride and/or oxynitride based phosphors, e.g. Sr2Al2Si10N14O4:Eu2+ have high efficiencies and chemical as well as thermal stability, these materials need to be prepared using complex and expensive synthesis conditions. Fluoride phosphors, such as K2TiF6:Mn4+, K2SiF6:Mn4+, NaYF4 doped with Yb3+, Er3+, Tm3+, are not very stable materials in air and humidity. Oxyfluorides are more stable compounds than fluorides with respect to hydrolysis. The presence of fluorine in the structure positively affects the photoluminescence properties of the phosphors: due to the ‘softer’ phonon modes at lower energies associated with the fluorine atoms in the lattice, thermal quenching is often reduced. Studies have been done on the PL properties of rare earth cation (Eu3+, Ce3+, Tm3+, Tb3+) doped Sr3AlO4F that show strong line emissions. The low phonon energy in the fluoride environment decreases the possibility of non-radiative transfers of electrons between energy levels of doped rare earth elements. Im et. al. have shown that the photoluminescence intensity of the oxyfluoride compounds Sr2.975−xBaxCe0.025AlO4F is about 150% that of commercial YAG:Ce3+ and its quantum efficiency at room temperature was near 95% due to its lower thermal quenching. One disadvantage of this oxyfluoride family is that the materials degrade albeit slowly when in contact with moisture. However dry preparation methods and proper handing can mitigate this problem.
Magnesium fluorogermanate is an oxyfluoride studied previously whose structural units are the same as those present in NaCa2GeO4F namely F− anions and GeO4 tetrahedral units. Magnesium germanate phosphors containing only GeO4 tetrahedral subunits were discovered as early as 1936 by Leverenz, who prepared weakly luminescent Mn4+ (4A2g to 4T1g and to 4T2g transitions) doped meta-magnesium germanate (MgGeO3) and ortho-magnesium germanate (Mg2GeO4). In 1948, Williams improved the luminescence efficiency of Mg1.99Mn0.01GeO4 5 fold when synthesizing Mg3.99Mn0.01GeO4. This phosphor emits intense red light under UV excitation and its efficiency does not change at temperatures as high as 300° C. making it suitable for use in compact fluorescent lamps (CFL)s based on high pressure mercury lamps as described in U.S. Pat. No. 2,447,448. In 1950, Thorington reported that the photoluminescent efficiency of Mn4+ activated magnesium germanate phosphor was doubled when introducing fluorine into the material resulting in the formation of a magnesium fluorogermanate. In 1972 Bless et. al. described this material to be Mg28Ge7.5O38F10 and its structure was found to be isomorphous with Mg28Ge10O48, having an orthorhombic, space group Pbam, a=14.343(1), b=10.196(1), c=5.9075(4) Å. In Mg28Ge7.5O38F10, there are some vacancies in the Ge sites when compared to the Mg28Ge10O48 structure and the F− anions substitute for some O2− sites for reasons of charge compensation. Ge4+ cations are tetrahedrally coordinated by oxygen and located on two sites. The Mn4+ activator occupies both the octahedral Ge4+ sites and the Mg2+ sites. This phosphor was subsequently used by Westinghouse Electric Company.
The synthesis of NaCa2GeO4F was first reported by Schneemeyer et al. in 2001 and its structure is described as a hexagonal antiperovskite. NaCa2GeO4F can be written as X3AB with A=GeO44−, B=F− and X=Na+, Ca2+ to emphasize its relationship to the perovskite structure. An antiperovskite is a perovskite where the cation and anion sites are interchanged. NaCa2GeO4F, has an orthorhombic unit cell with lattice parameters, a=5.362(2) Å, b=7.328(3) Å, c=12.681(4) Å, V=498.3(3) Å3, space group Pnma (No. 62), Z=4.
Viewing down the a axis of the NaCa2GeO4F structure reveals chains of isolated GeO4 tetrahedra as shown in FIG. 1a. Chains of face-shared fluorine-centered octahedra with the calcium/sodium ions (FM6, M=Ca/Na) are formed along the c axis (FIG. 1b). The Sr3XO4F (X=Al, Ga)-type materials have an antiperovskite structure where the F− ions are octahedrally coordinated by 6 Sr2+ ions which are then connected by corner-sharing and the (XO4)5− ions (X=Al, Ga) occupy the A-site within the voids of a ReO3 network. To emphasize the antiperovskite structure these materials can be written as (XO4)FSr3. However, instead of face sharing as in NaCa2GeO4F, Sr3XO4F structures have corner sharing octahedra. There is an axial elongation in the FSr6 octahedra and a ˜18° rotation about the c axis (FIGS. 1c and 1d). This is a0a0c− tilt system according to Glazer notation.
However, a need exists for further development of materials that can be used as phosphors in lighting applications, particularly in LEDs.